A method of ionizing a molecule to be analyzed and moving it through gas at a generally atmospheric pressure to analyze the molecule to be analyzed based on a difference in traveling time due to a difference in mobility of ions in the gas is known and called Ion Mobility Spectrometry (IMS).
An ion mobility analyzing apparatus employing ion mobility spectrometry has a cylindrical ion chamber in which ions are generated and a cylindrical drift tube in which ions are moved, wherein the ion chamber and the drift tube are connected in series, and an ion gate is provided at a boundary of the ion chamber and the drift tube. Ions to be analyzed, which have been generated in the ion chamber and have passed through the ion gate at a predetermined timing, are moved in the drift tube filled with gas at a generally atmospheric pressure, by an electric field formed in the ion chamber. A time is measured which is required for the ions to be analyzed to pass through the ion gate to reach the ion detecting electrode that is provided at the opposite end of the drift tube. A potential difference of several tens kilovolts is applied between both ends of the ion chamber and the drift tube in order to form an electric field that moves ions. Note that a value of the applied voltage varies depending on the sizes of the ion chamber and the drift tube. (See Patent Literature 1.)